Python from F#

python.fsx

// make sure codegen.py (https://github.com/CensoredUsername/codegen/blob/master/codegen.py) is in the same folder
// and you've added the IronPython package

open System.IO
open IronPython.Hosting
open IronPython.Runtime
open Microsoft.FSharp.Reflection

let engine = Python.CreateEngine()

let pythonPath = [|"/usr/lib/python2.7"
                   Directory.GetCurrentDirectory()|]
engine.SetSearchPaths(pythonPath)

// based on https://docs.python.org/2.7/library/ast.html
type AST =
    | Compare of AST * AST seq * AST seq
    | Call of AST * AST seq * AST seq * AST option * AST option
    | BinOp of AST * AST * AST
    | If of AST * AST seq * AST seq
    | IfExp of AST * AST * AST
    | Add | Sub | Mult | Div
    | Eq | NotEq | Lt | LtE | Gt | GtE | Is | IsNot | In | NotIn
    | Num of obj
    | Name of string

let code = IfExp(
            Compare(Name "a", [Gt], [Name "b"]), 
            Call (Name "some_function", [Num 100], [], None, None),
            Call (Name "some_other_function", [Num 50], [], None, None))

let ast = engine.ImportModule("ast")

/// Given a name and an array of arguments returns a Python AST node instance
/// 
/// This function handles some special casing and heuristics as well
let pythonAstNode name args =
    // look up ast type in ast module by name
    let astType = ast.GetVariable(name) :?> Types.PythonType
    match name with
    // Name takes an extra argument that we always want to be null for now
    | "Name" -> engine.Operations.Invoke(astType, Array.append args [|null|])
    // invoke the ast type on the provided arguments to return a new instance
    | _ -> engine.Operations.Invoke(astType, args)

/// Convert an F# sequence into a Python list
let toList<'a> (s:'a seq) =
    let l = List()
    for v in s do
        l.Add(v) |> ignore
    l    

/// Given an instance of our AST union (or sequences or options thereof)
/// return a Python AST node instance, Python list, or null
let rec toPythonAst (a:obj) =
    match a with
    | :? AST ->
        match FSharpValue.GetUnionFields(a, typeof<AST>) with
        | case, x -> pythonAstNode case.Name (Array.map toPythonAst x)
    | :? seq<AST> as s -> toList (Seq.map toPythonAst s) :> obj
    | :? option<AST> as o -> 
        match o with
        | Some v -> toPythonAst v
        | None -> null
    | _ -> a

let codegen = engine.ImportModule("codegen")
let codegenToSource = codegen.GetVariable("to_source") :?> PythonFunction
let pythonCode = toPythonAst code
printfn "%A" (engine.Operations.Invoke(codegenToSource, pythonCode))